124 BOTANY. 
their corners; finally only the centres of the rounded ends 
are left in slight contact, which soon breaks, 
261. The sexual reproduction is well illustrated in Spi- 
rogyra, one of the principal genera. At the close of 
their growth in the spring, the cells push out short tubes 
from their sides, which extend until they come in contact 
with similar tubes from parallel filaments (4, Fig. 56). 
Upon meeting, the ends of the tubes flatten upon each 
other, the walls fuse together and soon afterwards become 
absorbed, thus making a channel leading from one cell to 
the other (B, Fig. 56). Through this channel the proto- 
plasm of one cell passes into the other, and the two unite 
into one mass, which becomes rounded and in a short time 
secretes a wall of cellulose around itself (Fig. 56, B and C). 
The resting spore thus formed is set free by the decay of 
the dead cell-walls of the old filament surrounding it; it 
then falls to the bottom of the water, and remains there 
until the proper conditions for its growth appear. 
262. The germination of the resting spore is a simple 
process. The inner mass enlarges and bursts the outer hard 
coat; it then extends into a columnar or club-shaped mass, 
gradually enlarging upward from its point of beginning; 
after a while a transverse partition forms in it, and this is fol- 
lowed by another and another, until an extended filament 
is formed. 
‘263, The Black Moulds (Mucorini) are saprophytic and 
some times parasitic plants; they are composed of long 
branching filaments (hyph@), which always form a more 
or less felted mass, the mycelium ; when first formed the 
hyphe are continuous, but afterwards septa are formed in 
them at irregular intervals. The protoplasmic contents of 
the hyphx are more or less granular, but they never de- 
